1. Field of the Invention
The present invention relates to a process for removing water vapor from water vapor-containing gas utilizing a semi-permeable membrane. More specifically, it relates to a process for removing water vapor from water vapor-containing gas by flowing a dry condensable sweep gas on the downstream side of a semi-permeable membrane while flowing water vapor-containing gas on the upstream side of the membrane. Water vapor permeates from the upstream to the downstream side of the membrane. The condensable sweep gas containing water vapor is collected and condensed into an organic phase and an aqueous phase, which are then separated.
Many natural gases contain a significant amount of water vapor when the gases are produced from wells. It is well known that methane, ethane, and other hydrocarbons as well as carbon dioxide will form hydrates, solid ice-like materials, when conditions are such that liquid water is in contact with high pressure natural gas streams. Such hydrate formation can plug natural gas pipelines, preventing flow. Also, if liquids condense from the gas phase which contain water, significant corrosion of the pipeline material can occur. This is particularly so where carbon dioxide is also present in the gas stream. In addition to these problems, the formation of liquids in a pipeline can cause collection of these liquids in low points in the line which can cause slugging or unsteady flow which is also undesirable. It is therefore desirable to have a means for removing the water vapor from natural gas at the well site, and on off-shore wells, on the platform itself.
Air, oxygen, and nitrogen used in industrial processes often contain excessive amounts of water vapor which may have a detrimental impact on the processes being carried out. Thus it is desirable to have an effective and efficient means of removing water vapor from air, oxygen, nitrogen, and other industrial gases.
This invention is concerned with a compact, effective, efficient, and economical process for dehydrating gases. It is particularly useful for removing the water vapor from natural gas and/or removing water vapor from air.
2. The Prior Art
In the past, semi-permeable membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, water vapor, carbon dioxide, methane, and light hydrocarbons. Particular applications have included the use of membranes to dehydrate natural gas and air.
Several processes have been described in the prior art for removing water vapor from water vapor-containing gas. See for example U.S. Pat. Nos. 3,735,558; 3,822,202; 4,378,400; and 4,440,643.
A process for removing water vapor from water vapor-containing gas is also described in U.S. Pat. No. 4,718,921. In this patent, it is disclosed that water vapor-containing gas can be separated into a fraction having an increased content of water vapor and a remaining fraction having a decreased content of water vapor by a gas separating device which has at least one gas separating membrane having gas feed and delivery surfaces, in such a manner that (1) a water vapor-containing feed gas is fed to a feed side of the gas separating device: (2) the feed gas is flowed along the gas feed surface of the gas separating membrane to allow a fraction of the feed gas to permeate through the gas separating membrane: (3) a drying gas containing 300 ppm or less of water vapor is fed to a delivery side of the gas separating device: (4) the fed drying gas is flowed along the delivery surface of the gas separating membrane to promote the permeation of water vapor through the gas separating membrane: (5) the permeated gas fraction is collected together with the flowing drying gas at the delivery side of the gas separating device and (6) the remaining gas fraction not permeated through the gas separating membrane and having a decreased content of water vapor is recovered from the feed side of the gas separating device. The preferred drying or sweep gas disclosed in the patent is argon, although other gases such as nitrogen and neon gases are said to be suitable for use. A gas separating device for accomplishing these purposes is described in FIG. 1 and 2 of the patent.
U.S. Pat. No. 4,844,719 discloses a method for the desiccation of a water-containing gas which comprises bringing the gas into contact with one side of a membrane formed of a fluorine type copolymer containing specific repeating units, and either bringing a dry purge gas into contact with the other side of the membrane or reducing the pressure on the other side of the membrane, thereby removing water from the water-containing gas. The dry purge gas used in the process can be any one of a number of gases, including hydrogen, helium, and argon.
Other commonly used sweep gases, or purge gases as they are sometimes referred to, include air, carbon dioxide, and nitrogen.
It would be desirable in the art to provide a process for recovery of water vapor from water vapor-containing gases using a sweep gas which is inexpensive, easily condensed, possesses low water solubility in the condensed phase, and can be easily recycled if desired. This invention is concerned with such a process.